This invention relates to catalysts for hydrotreating residual petroleum oil, to methods of making the catalyst and more particularly to methods of tailoring the pore size distribution of the catalyst to the measured size distribution of sulfur, nickel and vanadium containing molecules in the residual oil to produce desired hydrotreating of the residual oil.
Residual petroleum oil fractions such as those heavy fractions produced by atmospheric and vacuum crude distillation columns, are typically characterized as being undesirable as feedstocks for most refining processes due primarily to their high metals and sulfur content. The presence of high concentrations of metals and sulfur and their compounds precludes the effective use of such residua as chargestocks for cracking, hydrocracking and coking operations as well as limiting the extent to which such residua may be used as fuel oil. Perhaps the single most undesirable characteristic of such feedstocks is the high metals content. Principal metal contaminants are nickel and vanadium.
Sulfur is also undesirable in a process unit chargestock. The sulfur contributes to corrosion of the unit mechanical equipment and creates difficulties in treating products and flue gases. At typical cracking conversion rates, about one half of the sulfur charged to the unit is converted to H.sub.2 S gas which must be removed from the light gas product.
Increasingly, residual oil is being upgraded into lighter petroleum products, notably transportation fuels, by hydrotreating the residual oil. This treating of residual oil in the presence of a catalyst and hydrogen to transform it into lighter petroleum products is difficult because of the aforementioned presence of sulfur, nickel and vanadium in the molecules of the residual oil.
Catalysts having different size pores have been used to hydrotreat residual oil. For example, it has been recognized that large pore catalysts remove metals faster than small pore catalysts whereas small pore catalysts remove sulfur faster than large pores.
Two layer catalyst beds have been used to hydrotreat residual oil. For example, U.S. Pat. No. 3,985,643 Milstein describes a hydrotreating process in which two beds are used to treat residual oil.
In the prior art, catalysts having different catalyst pore size distributions have been used. U.S. Pat. Nos. 3,876,523 Rosinski et al and 3,891,541 Oleck et al describe methods for preparing catalysts having specific pore size distributions. Generally, any given catalyst is a compromise if it is used to process all residual oils. While a catalyst having a given pore size distribution may be good for one residual oil, it will not be as effective for other residual oils.
"CONFIGURATIONAL DIFFUSION EFFECTS IN CATALYTIC DEMETALIZATION OF PETROLEUM FEEDSTOCKS" by J. C. Spry, Jr. and W. H. Sawyer, a paper presented at the 68th Annual AICHE meeting, Nov. 16-20, 1975, describes the analysis of residual oils by gel permeation chromatography. This paper also describes a computer model for defining catalyst performance in terms of catalyst properties. The paper concludes that there is an optimum catalyst pore size which is dependent upon the molecular size of the reactant. However, there is no disclosure of tailoring the catalyst pore size distribution to the measured characteristics of the residual oil to be processed.
Accordingly, it is an object of this invention to use different catalysts having different pore size distributions to produce desired hydrotreating of the residual oil having measured characteristics.